This is a non-concurrent prospective study to determine whether persons in the meat industry who are occupationally exposed to oncogenic viruses which cause cancers of the hemopoietic/lymphatic systems in cattle and chicken, are at risk of developing similar cancers. A preliminary study conducted among this occupational group indicates an excess of deaths from tumors of the hemopoietic and lymphatic systems. There is therefore a need for such a finding to be corroborated, by a more detailed study employing a more rigorous design and methodology. The study population will consist of persons who have ever been members of the Amalgamated Meatcutters Union (Local 117) in Baltimore, Maryland since 1950 (estimated size = 30,000). The subjects will be identified from Union records and followed-up by various methods to ascertain their status as to being dead or alive as of the 31st of December 1980. Causes of death will be ascertained from death certificates, and cause-specific standardized mortality ratios (SMRs) and proportional mortality ratios (PMRs) for all cancer deaths will be determined. The comparison group for the SMRs and PMRs estimation will be the United States general population; in addition other reference populations will be employed for comparison. A questionnaire will be administered to all study participants to obtain certain required information. A case/control study will be conducted in order to control for potential confounding factors associated with tumors of the hemopoietic/lymphatic systems. Other potential confounding factors for other cancers will also be controlled for in the analysis.